How can I start learning mathematics for machine learning?
You should begin from Dr Andrew Ng machine learning course on Coursera. It's probably the most popular course for newcomers in machine learning. It's a free course.

You should also grab "Elements of Statistical Learning" ebook PDF. It's a free book.

You may want to focus on:


Regression
Cross validation
Bias-variance tradeoff
Decision surface
Gradient descent


And more...